1. Field of the Invention
The invention is related to the field of communications, and in particular, to communication systems that mitigate outages caused by polarization mode dispersion.
2. Description of the Prior Art
Optical fibers are used to carry large amounts of communication traffic throughout the world. In a typical configuration, an optical transmitter is connected to an optical receiver by an optical fiber. The optical transmitter receives a communication signal and emits light pulses that represent the communication signal into the optical fiber. The light pulses propagate within the optical fiber from the optical transmitter to the optical receiver. The optical receiver detects the light pulses and provides the corresponding communication signal.
In many cases, the optical transmitter and receiver comprise Wavelength Division Multiplex (WDM) devices that simultaneously transmit and receive light pulses at multiple optical wavelengths. With WDM, the optical transmitter simultaneously receives multiple communication signals and simultaneously emits corresponding light pulses at different optical wavelengths, where each communication signal uses a different optical wavelength. The optical receiver separates the light pulses by wavelength to differentiate the communication signals. Thus, multiple communication signals may be simultaneously transferred over an optical fiber at different respective wavelengths.
The optical fiber attenuates the optical pulses as they traverse the fiber. This attenuation is sometimes referred to as loss that can be quantified in decibels. To prevent attenuation from destroying the optical pulses, discrete optical amplifiers are inserted in between the optical transmitter and receiver on long fiber routes. The discrete optical amplifiers boost the power of the optical pulses, so the optical receiver may properly detect the pulses and provide the corresponding communication signal. Unfortunately, these discrete optical amplifiers also introduce noise into the light pulses and amplify pre-existing noise.
If the discrete optical amplifiers cannot sufficiently boost the light pulses to overcome the attenuation, then a regenerator site must be installed in the middle of the optical fiber route. The regenerator site requires WDM equipment to convert each wavelength to an electrical signal. The regenerator site requires expensive line cards to condition the electrical signals. The regenerator site requires additional WDM equipment to regenerate the wavelengths from the conditioned electrical signals.
Distributed Raman amplifiers also boost the power of light pulses on an optical fiber. The distributed Raman amplifier pumps additional light onto the fiber—sometimes in the opposite direction of the light pulses that represent the communication signal. Under the right conditions, energy from the pumped light is transferred through the optical fiber structure to the light pulses that represent the communication signal. Noise generated by the Raman amplifier is attenuated by fiber loss as well as signal power. Thus, the distributed Raman amplifiers could have better Optical Signal to Noise Ratio (ONSR) than discrete amplifiers.
Recently, optical wavelength converters have been developed. The optical wavelength converters receive light pulses at a first wavelength and transmit corresponding light pulses at a different wavelength. The optical converters also remove some of the noise introduced or amplified by the discrete optical amplifiers. As a result, the converted pulses could have a higher OSNR.
The optical fibers may degrade the light pulses due to a fiber impairment called Polarization Mode Dispersion (PMD). When PMD exists, the optical fiber propagates different polarization modes of the light pulse at different speeds. Note that although the speed of light in a vacuum is constant, optical fibers are typically not in a vacuum, so the speed of light in an optical fiber can vary. As the light pulse propagates down an optical fiber that exhibits PMD, the optical pulse spreads out in time and lessens in amplitude, since the polarization modes of the optical pulse travel at different speeds. The faster mode of the pulse takes the lead and slower mode of the pulse falls behind. By the time that the optical pulse reaches the optical receiver, the ideal square wave shape of the pulse may resemble more of an elongated bump. When PMD is severe, the optical pulse spreads to the point of being unrecognizable. The optical receiver cannot accurately detect these unrecognizable light pulses, and as a result, the optical receiver provides a recovered communication signal with significant bit errors. If the Bit Error Rate (BER) reaches a set threshold, then a PMD outage has occurred. To avoid PMD outages, higher OSNR is required.
Unfortunately, optical wavelength converters and distributed Raman amplifiers have not been effectively deployed to mitigate PMD outages.